Commercial processes for the production of acetic acid are known. Several conventional processes involve the catalyzed carbonylation of methanol with carbon monoxide. Examples of these conventional processes include those described in U.S. Pat. Nos. 3,769,329, 5,001,259, 5,026,908, and 5,144,068, which are hereby incorporated by reference.
One of the most widely used processes for the manufacture of acetic acid is the Monsanto process, which involves carbonylating methanol in the presence of rhodium, methyl iodide, methyl acetate and water. The product is suitable for many conventional purposes. The acetic acid produced via the Monsanto process, however, suffers from iodide contamination. Another conventional methanol carbonylation process is the Cativa™ process, which is discussed in Jones, J. H. (2002), “The Cativa™ Process for the Manufacture of Acetic Acid,” Platinum Metals Review, 44 (3): 94-105. Although fewer iodides often may be present due to the use of catalyst promoters, iodide contamination is still an issue with the crude acetic acid products of the Cativa™ Process.
Macroreticulated strong acid cationic exchange resin compositions are conventionally utilized to reduce iodide contamination. Suitable exchange resin compositions, e.g., the individual pellets thereof, comprise both sites that are functionalized with a metal, e.g., silver or palladium, and sites that are non-functionalized. Exchange resin compositions that have little or no metal-functionality do not efficiently remove iodides and, as such, are not conventionally used to do so. Typically, metal-functionalized exchange resins are provided in a guard bed and a stream comprising the crude acetic acid product is passed through the guard bed. In the guard bed, the iodide contaminants contained in the crude acetic acid product attach to these metal-functionalized sites and are removed from the acetic acid product stream. The non-metal-functionalized sites generally do not capture iodides.
The metal-functionalization of exchange resin compositions often involves significant processing and expense, often costing orders of magnitude more than resins that are not metal-functionalized. Often the process steps associated with the functionalization varies very little with regard to the actual amount of metal that is deposited on the exchange resin. For example, the processing necessary to functionalize 50% of the active sites of a quantity of exchange resin is quite similar to the processing necessary to functionalize 10% of the active sites of the same quantity of exchange resin. Because the entire quantity of exchange resin requires processing, however, both the 50%-functionalized exchange resin and the 10%-functionalized resin require significantly more processing than the same quantity of non-functionalized resin.
In addition to iodide contaminants, metals from the walls of the vessels used in the acetic acid production system often corrode and dissolve into the crude acetic acid product compositions. Thus, conventional acetic acid product streams often comprise corrosion metal contaminants as well as iodide contaminants. These corrosion metals are known to interfere with the carbonylation reaction or accelerate competing reactions such as the water-gas shift reaction. Typically, these corrosion metals may be removed from the process streams by passing the streams through resin guard beds comprising standard, non-metal-functionalized cationic exchange resins. It is not necessary nor economically practical to use an expensive metal-functionalized exchange resin to remove corrosion metals.
In a case where an exchange resin with individual pellets each comprising both functionalized and non-functionalized sites is utilized, however, the corrosion metals may detrimentally clog the metal-functionalized sites of the exchange resins. As such, the clogged sites are unable to capture/remove the iodide contaminants. As such, the lifetime of the functionalized resin, with regard to iodide removal, is shortened by the presence of corrosion metals. Often a pre-determined portion of the sites on each of the pellets of the exchange resin composition are functionalized, thus leaving the remainder of the sites available for corrosion metal removal. As a result, the non-functionalized active sites attract the corrosion metals while the functionalized sites remain available for iodide removal. Although this technique may improve the lifetime of exchange resins, the partial functionalization of the pre-determined portion of sites on each pellet requires significant processing and resources.
Thus, the need exists for a process for preparing an exchange resin composition comprising a predetermined portion of functionalized sites wherein the quantity of metal functionalized resin pellets in the exchange resin composition is reduced. By reducing the quantity of metal functionalized exchange resin pellets in the exchange resin composition, the overall processing required to prepare the exchange resin composition may be lessened.